The receiver of many modern sporting rifles comprises an upper receiver portion and a lower receiver portion. The upper receiver portion houses, among other components, the barrel, the bolt carrier and the charging handle. The lower receiver portion includes the firing mechanism the magazine and the rifle stock. Many rifle designs use two pins, a forward pin (positioned near the barrel breech) and a rear pin (positioned near the rifle stock), which engage the upper and lower receiver portions and hold them together. In a popular rifle design the forward pin is mounted in a pivot bearing, which permits the upper and lower receiver portions to pivot relatively to one another between an open and a closed configuration. In this design the rear pin may be mounted on the lower receiver portion and engage a catch mounted within the upper receiver portion. The rear pin is designed to be removed from the lower receiver portion or otherwise displaced from engagement with the catch to permit the upper and lower receiver portions to pivot relatively to one another about the forward pin.
The many advantages of the pivotable upper and lower receiver designs include ease of cleaning the rifle, ease of repair, and access to the magazine, which is especially useful for reloading if the magazine is fixed in the lower receiver and the only access to it is when the upper and lower receiver portions are in the open configuration. These advantages, however, are not uniformly realized because it is often difficult to remove or displace the rear pin to disengage it from its catch and free the upper and lower receiver portions from one another. The difficulty may arise, for example, because of binding between the rear pin and the lower and upper receiver portions. Tolerances and misalignments vary the force required between different rifles to overcome this binding; some rear pins require a tool to displace them into and out of engagement. There is clearly an opportunity to improve the design of modern sporting rifles and better realize the advantages of pivotably attached upper and lower receiver portions.